This invention relates to methods and apparatus for reducing noise in video signals. It is especially, but not soley, applicable to video signals obtained from photographic or cinematographic film for example in a flying spot or photoconductive telecine, particularly when aperture correction is employed. It is particularly, but not solely, applicable to a high definition telecine, where the noise is higher and more aperture correction is usually required.
To appreciate the background behind telecines aperture correction and noise reduction, it is useful to consider photoconductive telecine. These telecines use a photoconductive camera where the major source of noise is the head amplifier. This noise source is constant at all levels of amplitude at the amplifier output. In the camera circuitry, the output of the head amplifier forms the input to the gamma corrector which corrects the signal to compensate for the receiver CRT characteristic. The gamma corrector has a nominal power law of 0.4 which results in a higher differential gain at black level as compared to white level. This causes a particular problem as it has the effect of amplifying noise in the signal to a greater extent at black level than white level. Unfortunately, the human eye is more sensitive to noise in black areas than whites.
Photoconductive cameras conventionally use aperture correction to compensate for various losses. Aperture correction increases high frequencies to compensate for fine detail contrast reduction caused by the finite size of electron spots. Without aperture correction applied, the noise level in a gamma corrected signal is not normally significant. However, after aperture correction the noise at black usually becomes observable. As a result, it is very common practice to reduce or eliminate aperture correction as the signal level approaches black, and this practice is called "level dependant aperture correction". Two examples of "level dependant aperture correction" are shown in FIGS. 1 and 2, the only difference between the two systems being whether the aperture correction is applied before or after gamma correction. Level dependant aperture correction has the effect of reducing the resolution in the blacks, but it has been found that the human eye is less likely to detect the loss of resolution than the noise.
The major noise source in a flying spot telecine is the photomultiplier. Noise amplitude is proportional to the square root of the light input and is therefore zero at black. After gamma correction the noise is approximately constant over the signal range and it is not normally necessary to apply level dependant aperture correction.
This situation is no longer true when negative film is used. Then, the gamma corrector has a power law of -0.8 and the noise distribution is no longer uniform but of much greater amplitude in the whites. The level of noise is barely acceptable in present television systems and will certainly not be acceptable with HDTV systems.
The difference in noise levels between telecines operating with positive and negative films may be better appreciated from the following examples:
The signal to noise ratio in a flying spot telecine is usually only measured in the positive mode at a signal level equivalent to a scene brightness of one tenth of the peak brightness, at a gamma of 0.4. A typical good figure obtained in practice is 50 db where 40 db is near the acceptable limit.
Using the 50 db figure as a basis, the signal to noise figures at peak brightness, one tenth peak, and one hundredth peak for positive film are:--52 db, 50 db, and 48 db respectively to the nearest db.
For negative films, at a gamma of -0.8 and allowing for the loss of light due to the orange mask in the green channel, the figures obtained are 31 db, 44 db and 57 db respectively. Thus, there is clearly a wide variation in signal to noise from black to white. A gamma of -0.8 in the telecine is necessary for negative films so that the viewer of a TV receiver perceives a picture with the same brightness graduation as the original scene, and the orange mask in negative films is used to aid printing from negative films.
We have appreciated that the gamma characteristic in the telecine can have a marked effect on the signal to noise ratios, the noise normally being in the upper frequency band due to aperture correction. We have also appreciated that the correct gamma characteristic must be applied in the telecine or camera to reproduce the correct brightness graduation, but this correct characteristic need only apply to the low frequency components of the signal. Therefore, in accordance with one aspect of the invention, the high frequency components of the signal could be processed with a different gamma and indeed it is not necessary to employ a perfect gamma curve but rather it is possible to use the most appropriate non-linear characteristic that produces the greatest reduction of noise.